It shall be the duty of the state treasurer to render a statement to the commissioners of the sinking fund , on the first business day of January, April, July and October in each and every year, giving the balance remaining in the sinking fund in excess of the amount required to pay the interest on the public debt, whereupon the commissioners of the sinking fund shall authorize the purchase, by the fiscal agent of the state, of the loans of the commonwealth at the then market rates: Provided, That all such investments shall immediately be cancelled, by said agent, and a certificate of the same forwarded to the commissioners of the sinking fund in like manner, as if the said loan had been redeemed by the commonwealth.
72 P.S. § 3674